The background of the invention is that is has proved possible by means of a relatively moderate heat treatment substantially to improve the keeping qualities of the kind of food products which are not deep frozen or sterilized but are maintained in a fresh state until prepared for consumption. Such food products are for instance sliced meat products, such as sausage, ham, etc. One reason for the fact that improved keeping qualities can be achieved particularly in connection with sliced food products is assumed to be that a small amount of bacteria often may be added in connection with the slicing operation and the subsequent packing, which puts a limit to the keeping qualities of the food products. It has been shown that the effect of these bacteria may be reduced substantially if the food products are subjected to a relatively moderate heat treatment immediately after the slicing operation. The invention thus aims at enabling in a rapid and simple way improvement of the keeping qualities of for instance such food products.
Apparatus has previously been known for rapid heat treatment of packed food products by means of microwave energy. However, known apparatus of this kind, as a rule, have a relatively complicated construction and often include sophisticated equipment for accomplishing an even heat treatment effect in the packed products.
Thus, US-A-3.809.844, US-A-3.809.845 and US-A-3.814.889 describe plants in which food products are subjected to an accurately controlled heat treatment before they are heated by means of microwave energy and, moreover, are cooled during the microwave heating by means of a surrounding fluid in order not to get too high a surface temperature.
EP-A1-0.287.760 describes a plant in which packed food products are placed at accurately determined intervals on an endless belt, on which they are moved intermittently and subjected to microwave heating in different heat treatment zones. Microwave applicators arranged at the heat treatment zones are formed in a particular way with regard to the shape of the food products and are arranged to be moved towards and away from the food products during the microwave heating, when the endless belt keeps the food products stationary in the heat treatment zones.
EP-A1-0.351.491 describes an apparatus in which packed food products are placed in several parallel rows on an endless belt and are moved by the belt under a number of microwave applicators, each one of which is arranged for heat treatment of the food products in only one of the rows. The food products have to be placed very accurately on the endless belt to be subjected to the intended heat treatment under the microwave applicators. At least one of the microwave applicators for each row of food products has the shape of an elongated wave guide extending along the movement path of the food products and has several slots distributed along said movement path, through which the microwaves may radiate and hit the passing food products.
None of the above described known plants are, in practice, suitable for resolving the particular problem to which the present invention is directed. The known plants are either too complicated and expensive or are imperfect from a functional point of view.
An object of the present invention is, thus, to provide a method and a plant for rapidly performing a relatively moderate heat treatment of relatively thin separate food products for improvement of their keeping qualities. It is important that the heat treatment be performed simply and cheaply, i.e. a plant for the performing of the heat treatment should be inexpensive and be very easy to use. For instance it should be simple to feed the food products into the plant. Furthermore, the plant should be usable without costly reconstruction for heat treatment of food products having somewhat different shape and size.